1. Field of the Invention
The invention herein relates to operation of manufacturing processes and equipment in which a valuable or environmentally hazardous gas is utilized. More particularly, it relates to methods for recovery from and subsequent reuse of that gas in such processes and equipment.
2. Description of the Prior Art
There are numerous manufacturing processes in which gases are used as reactants or to provide a process atmosphere, but in which a significant amount of free excess gas remains upon completion of the process. If the gas is both inexpensive and non-hazardous to the environment, the excess gas can simply be vented to the ambient environment. Typical examples of such gases are air, oxygen, nitrogen, water vapor and, in some cases, argon.
In many other processes, however, the gas that is present in the process, either as initially supplied to the process vessel or chamber or as generated during the process, is an expensive gas, a gas in limited supply or a gas which poses some type of environmental hazard, including hazards to humans and animals. The manufacture of xenon-filled light bulbs represents a typical example of such a process. The light bulb components are first assembled under ordinary ambient conditions of the manufacturing facility (i.e., in air) with a small opening left in the glass envelope. The assembled bulbs are then placed in a gas-tight, high-temperature-capable chamber. The chamber is sealed and all of the air is evacuated, following which the chamber is filled with xenon gas. Some of the xenon gas fills the interior of the glass envelope of each bulb, and subsequent heating of the chamber causes the opening in the glass envelope to become sealed, trapping the xenon gas filler within the envelope of the bulb. The chamber and bulbs are then allowed to cool, so that the bulbs are ready for retrieval. There is, however, a large excess of xenon gas remaining in the chamber over that which is now sequestered by being sealed in the light bulbs. While the venting of xenon does not pose an environmental hazard, xenon is a rare and quite expensive gas and any quantity of xenon vented to the atmosphere is thereafter unrecoverable. Venting, therefore, represents a substantial loss of a valuable and costly resource, xenon gas, so the bulb manufacturers want to recover as much of the excess xenon as possible after each batch of bulbs is manufactured and recycle it for reuse in the manufacture of subsequent batches of light bulbs.
A few processes have been described in the prior art for recovery of xenon and similar gases. Such processes have commonly involved cryogenic steps, are quite complex and expensive to operate and the gas recovery is often relatively low. Consequently, such processes have generally been used only when the product being manufactured is of sufficiently high value that the cost of such gas recovery can be justified as part of the overall manufacturing cost. For relatively low cost items such as light bulbs, of course, such costs cannot be justified and the xenon gas has, in the past, usually been vented and lost.
Another type of gas recovery process has been described in European Patent Application 0 826 629 A2. In this process xenon, neon, krypton or mixtures thereof is said to be recovered from a process chamber by mixing with a purge gas such as hydrogen, steam (water vapor), carbon monoxide or dioxide, oxygen or a gaseous C.sub.2 -C.sub.6 hydrocarbon, followed by separation of the process gas and the purge gas. Such a process requires extensive gas separation facilities, such as membrane and zeolite separators, and leaves the process gas with residual purge gas contamination. Purge gases such as steam and the hydrocarbons can also condense on the interior surfaces of system piping, valves, pumps, etc., attacking and corroding metals, seals and other system components.
There have been attempts in the past to recover xenon gas from light bulb manufacturing and similar manufacturing processes by pumping the gas to storage and then pumping it back to the reaction chamber for subsequent reuse. However, such processes have proved to be unsatisfactory beyond very short time periods (i.e., over more than a few iterations of the manufacturing process), since the gas quickly becomes contaminated with foreign material during the repeated pumping cycles. This in turn causes deterioration of the successive batches of the manufactured products as the increasingly contaminated gas is repeatedly recycled back to subsequent runs of the manufacturing process.
In those processes where the gas being used or generated is one which is environmentally hazardous, venting of the gas following the manufacturing process is unacceptable and often illegal. In the past, one practice has been to remove the gas from the chamber and pass it to a reaction or neutralization process where the gas is chemically converted to environmentally acceptable materials or is sealed in tanks, cylinders and the like for storage and prevention of escape to the atmosphere. Where reuse of the gas has been contemplated, problems similar to those with xenon have been encountered, in that the gases have become contaminated with moisture or foreign materials to the extent that repeated recycling of the gases has resulted in progressive degradation of the products being manufactured.
Other processes have been used for recovery of some types of hazardous gases, such as scrubbing for acid-precursor gases such as the sulfur oxides. However, in addition to being expensive, such processes usually covert the gases into a different physical form or chemical composition, and therefore prevent the gas from being recycled and reused.